Apple co-founder Steve Jobs has stepped down from his position as CEO of the company and Tim Cook, Apple's chief operating officer, has been named to take the helm, Apple announced Wednesday.

"I have always said if there ever came a day when I could no longer meet my duties and expectations as Apple's CEO, I would be the first to let you know. Unfortunately, that day has come," Jobs wrote in a letter to Apple employees.

Jobs has wrestled with cancer in recent years and took an indefinite leave of absence in January to focus on his health. An Apple spokeswoman declined to elaborate on why Jobs resigned at this time. He submitted his resignation Wednesday and "strongly recommend[ed]" that Cook be elected CEO, Apple said.

Jobs has been elected chairman of Apple's board, effective immediately, and Cook has been made a board member, the statement said.

"I believe Apple's brightest and most innovative days are ahead of it. And I look forward to watching and contributing to its success in a new role," Jobs wrote in his letter.

As Apple's chairman, Jobs will continue to serve Apple "with his unique insights, creativity and inspiration," Art Levinson, chairman of Genentech and an Apple board member, said in a statement. "Steve's extraordinary vision and leadership saved Apple and guided it to its position as the world's most innovative and valuable technology company."

Jobs is widely admired for returning to the struggling company in the mid-1990s and turning it into one of the most successful technology companies in the world. Under his leadership, Apple developed the iPod, iPhone and iPad -- devices that have each defined their technology categories. Now, analysts wonder if the company can continue that trend without him.

Jobs' core strength has been in envisioning markets, such as tablets, said Roger Kay, president of Endpoint Technologies Associates. The company may have tried to anticipate Jobs' departure by planning a long pipeline of products for the future, he said.

But even if it can't anticipate whole new categories, it should be in a comfortable spot for the next few years. Apple is in an unassailable position in the smartphone and tablet markets and would need to make significant errors to falter in those areas, Kay said.

In addition, Cook already has extensive experience running Apple in Jobs' absence, and Jobs will still have a role at the company as chairman. "You are talking about a guy who will continue to have influence on Apple's products ... for years to come," said Michael Gartenberg, an analyst at Gartner.

And there is more to Apple than Jobs, he noted. He pointed as an example to Jonathan Ive, senior vice president for industrial design at Apple, who was the creative force behind the iPhone, iPad and iPod, and at one point a popular choice among Apple enthusiasts to replace Jobs as CEO. Another candidate considered a possible replacement was Phil Schiller, senior vice president of worldwide product market.

Jobs is "an icon and what he's done with Apple is something probably unprecedented in business," said IDC analyst Al Hilwa. "It will be a case study in business school books for decades."

This year's leave of absence was Jobs' second. In 2004 he underwent surgery to treat a rare form of pancreatic cancer and returned to work soon after. But in early 2009 Jobs took six months off to treat what he called a "hormone imbalance" that made him appear gaunt. During that leave he underwent a liver transplant.

Following the news of Jobs' resignation, Apple's stock was down more than 5 percent in after-hours trading.

(Robert McMillan, James Niccolai and Agam Shah contributed to this report.)